Anorexia nervosa usually has its onset in adolescence, a time which is already characterized by marked changes in height and body weight. Weight restoration is an important early goal in the treatment of anorexia and is accompanied by a reversal of the medical complications of the disease and improved mood, and is felt by many to be necessary for psychotherapy to be effective. However, there is a lack of consensus as to how to determine treatment goal weight in the growing adolescent when both height and weight are changing as part of normal development. Some people use the reappearance of periods as an indicator of return to health and researchers in the U.S. followed 56 adolescent girls, between the ages of 12 and 19 over a three-month period to see at what body mass index (BMI) their periods returned. They concluded that this was at a BMI between the 14th and 39th percentiles (see the link to Wikipedia for an explanation of percentiles) and recommended that this be used as a treatment goal weight.
Golden, Neville H. ... [et al] - Treatment goal weight in adolescents with anorexia nervosa: use of BMI percentiles International Journal of Eating Disorders 2008; 41: 301-306